1. Field of the Invention
The present invention relates generally to a wafer polishing apparatus, and more particularly to a wafer polishing apparatus which is used with a chemical mechanical polishing (CMP) method.
2. Description of Related Art
According to a conventional wafer polishing apparatus used with the CMP method, it is difficult to directly detect the stock removal of the wafer during polishing, and thus, the stock removal of the wafer is controlled by managing a polishing time. For this reason, a dummy wafer is polished at regular intervals to find the stock removal in accordance with the polishing time, and this stock removal is compared with a model stock removal as a basis. If the difference between them is within the permissible level, the polishing continues, and if the difference exceeds the permissible level, the polishing is stopped to dress or replace the polishing pad.
The wafer polishing apparatus disclosed in Japanese Patent Provisional Publication Nos. 6-79618 (corresponds to U.S. Pat. No. 5,205,082) and 8-229808 adheres the wafer to a carrier, and applies a pressure force to the carrier to press the wafer against the polishing pad, thus polishing the wafer.
According to another conventional wafer polishing apparatus used with the CMP method, the air is supplied into a space between the carrier and the wafer, and the wafer is polished while it is pressed against the polishing pad. Since it is difficult to directly detect the stock removal of the wafer, the sensor detects the movement amount of the carrier with the body of the wafer polishing apparatus being the basis, and the output of the sensor is regarded as the stock removal of the wafer. Thus, the stock removal of the wafer is detected indirectly.
The conventional wafer polishing apparatus, however, cannot correctly detect an end point (a target value) of the stock removal because the stock removal of the wafer is controlled by managing the polishing time.
Since the conventional wafer polishing apparatus polishes the dummy wafer to determine the timings for dressing and replacing the polishing pad, the yield of the wafer is deteriorated.
Moreover, since the normal polishing is stopped while the dummy wafer is polished, the throughput is lowered.
According to the polishing apparatus disclosed in Japanese Patent Provisional Publication Nos. 6-79618 and 8-229808, the carrier presses the wafer directly against the polishing pad. If there are some foreign matters such as polishing dust between the carrier and the wafer, the pressure force cannot transmit uniformly to the entire surface of the wafer, and the entire surface of the wafer cannot be polished uniformly.
According to the conventional wafer polishing apparatus which detects the stock removal of the wafer with the body of the wafer polishing apparatus being a basis, the body is expanded by heat which is generated during the polishing. For this reason, if the movement amount of the carrier is detected, the stock removal of the wafer cannot be detected correctly. Thus, the polishing end point of the wafer cannot be detected correctly.